1. Field of Invention
This invention relates to a new reciprocating compressor design. Specifically it relates to the use of auxiliary ports to handle additional suction and discharge pressures in a reciprocating compressor.
2. Description of the Prior Art
Current reciprocating compressors and linear compressors have a single suction valve and a single discharge valve. The piston draws gas through the suction valve during the down stroke. At the bottom of the stroke, the suction valve closes and allows the piston to compress the gas during the up stroke. Once the pressure inside the cylinder exceeds that of the discharge gas, the discharge valve opens and allows the gas to discharge from the cylinder.
A problem with this arrangement is that the compressor can only handle a single suction and a single discharge pressure. Many refrigeration and heat-pump applications need more that one suction or discharge pressure, which means that the lowest evaporating condition and the highest condensing condition set the pressures to the compressor. This situation leads to an inefficient system that sacrifices both efficiency and capacity. The other alternative requires the use of multiple compressors and staged arrangements that add to complexity and cost.
Gardner Voorhees in his 1905 patent (793,864) entitled "Multiple effect compressor," describes a valving arrangement that can improve capacity- and efficiency of reciprocating compressors in systems with multiple evaporating pressures. The basic idea is to add a port to the cylinder wall that can introduce gas at a higher suction pressure. The piston uncovers the port at the bottom of its stroke, which allows the higher-pressure gas into the cylinder. The higher pressure closes the main suction valve. Multiple auxiliary ports can be added to give additional suction pressures.
This multiple-effect system was commonly used in large ammonia compressors in the 1920's. These compressors usually ran at speeds of 100 rpm or slower, which is- extremely slow by today's standards. Typical applications were large ice plants, which used a second, higher-pressure evaporator to precool incoming water.
While the multiple-effect compressor systems gave significant efficiency and capacity advantages, changes in compressor design effectively eliminated their widespread use. The introduction of modern high-speed compressors in the 1930's and 40's greatly reduced physical size of compressors, which also reduced the value of the capacity advantage associated with multiple-effect systems. These higher speeds also greatly increased the potential wear problems associated with piston rings moving over an auxiliary port. Lower energy costs reduced the value of the efficiency advantages. These considerations have made the multiple-effect compressor simply a historical curiosity.
U.S. Pat. No. 4,332,144 makes use of similar ideas. This system uses a heat exchanger that evaporates refrigerant at an intermediate pressure to subcool refrigerant liquid before it reaches the main evaporator. This arrangement improves the efficiency and capacity of the system, but it does not describe a particular compressor design for accommodating the higher-pressure port.
The objective of the present invention is to improve upon the multiple-effect compressor. These improvements should eliminate the previous problems with piston rings sliding over an auxiliary port. They should also provide additional options that further improve the efficiency and flexibility of the compressor and the refrigeration system.